Electronics and programming interspersed at various levels of difficulty.

Sunday, March 30, 2014

Scosche myTrek - teardown and repair

Got this pulse monitor from eBay, advertised as broken. When paired with a phone it always showed 100 bpm. Searching a bit on the Internet saw that quite a few people had this problem as well ("stuck at 100 bpm").

The front has three buttons, the middle one being pairing/on/off and the left and right ones supposedly change the tracks on your phone media player. Controls seem very similar to the bulk of bluetooth headphones.
The backside has the sensor in the upper part - I suspect the pulse-oximeter type one - and two round pads for the charger dock.

It's clear now why the bracelet does not give any readings:

Due to bad engineering there is no proper strain relief for the wires between the sensor and the controller.
I'm inserting a link break here since this post a lot of pictures.

The cover comes off using only my fingernails, so I'm also wondering about their 'water resistant' claim.

The controller/remote is built as a sandwich construction: two boards with a battery between them.

The main chip is the bluetooth module: CSR 41C6 71AU K049CX, which I assume it's a serial-over-bluetooth solution. I have not tried connecting to it but it's in my pipeline.
The other chip is marked UC7061 BCPZ32 - an Analog Devices ARM7 microcontroller.

The bottom board is just a breakout board for the charging pads and sensor board.

The sensor is as simple as it gets, two leds and a phototransistor:

Through the wonders of a 100USD smartphone and digital zoom we can see a lot of details from the dies which are about 1sq.mm in area:

I tried soldering back the wires using a flex cable as extension but the original wires are too thin and fragile.

In the end I just took some ribbon wires from an IDE interface and soldered those, clearing also a bigger hole in the case:

It works!

Sandwiching it back together proves to be a bit of a problem so after taking this picture I cut the wires to a minimum folding length:

And then proceeded to make the case water-resistant with glue. Here's a picture of the clamps attacking the poor watch:

I don't remember, I think I zig-zagged some thin tape between them, afterwards they will stay flat anyway. Or I might have used hot glue if there was enough space. See this pic if you can figure it out: http://2.bp.blogspot.com/-r-Iil1mN_L0/UzgBx5ak3FI/AAAAAAAABFk/eK7AtUOgi3Y/s1600/IMG_1243.JPG